CM311a distortion

I recently purchased a CM311A and I'm experiencing distortion occasionally. I tested it with a pro-tools digi003 unit and it was fine. On the job however I get distortion unrelated to the pop filter as it happens pre-moisture. I use a Mackie 800 power head or a Phonic 600 and it seems to happen with both heads. I'm thinking something with the phantom power as both heads are phantom powered, but it's just a hunch. I'm a piano/vocalist, solo or small group, no loud screams unless I fall off the bench. Any ideas? Thanks

Here is a quick idea: put a 9V battery in the unit and turn on the battery switch and test it. If the distortion goes away, I would lean toward a phantom power problem. If it still has the same problem, then it it something in the microphone.